Field of the Invention
The invention relates to novel o-nitrophenol derivatives and o-nitrothiophenol derivatives, which are also jointly abbreviated to o-nitro(thio)phenol derivatives, and to a process for their preparation.
The preparation of high-temperature-stable polymers, such as polybenzoxazoles (PBOs) and their precursors, and the preparation of hydroxypolyimides requires bis-o-aminophenols (in this respect, see, for example, EP 0 264 678 B1 and EP 0 300 326 B1). These monomers can be prepared by reducing the corresponding bis-o-nitrophenols (in this respect, see EP 0 317 942 A2, SU 1 205 518 A and "Polymer Preprints" 34(1), 1993, pages 425 and 426).
Bis-o-nitrophenols can be prepared by nitrating bisphenols. However, this gives isomers and also polynitro compounds, which is very disadvantageous since, if the nitration is not complete, i.e. to 100%, and does not take place entirely isomer-free, i.e. nitration only taking place in the o-position to the hydroxyl group, then the reduction is in some cases followed by the formation of aminophenols which do not allow complete cyclization in the PBO precursor and considerably impair the properties of the polybenzoxazole.
It is also already known to prepare nitrophenol derivatives, i.e. bis(o-benzyloxynitrophenyl) compounds, for said purpose. This is achieved by nucleophilic substitution (see "Polymer Preprints", in loco citato). However, this process requires high temperatures, namely significantly higher temperatures than 100.degree. C. (solutions in dimethylacetamide/toluene are refluxed). However, high reaction temperatures promote side-reactions, which reduce the yield (it is a maximum of 73%) and make purification of the target product more difficult. In addition, the bis-o-aminophenols prepared in this way are not stable to oxidation.
In general, there is a lack of suitable aminophenols--and correspondingly also of nitrophenols--for the preparation of polymers which satisfy the greatly increased demands of microelectronics. In addition, the type of aminophenol employed has a strong effect on the property profile of the PBO precursor or polybenzoxazole prepared therewith. For example, not only the thermal, electrical and mechanical behavior, but also the solubility and hydrolysis stability and numerous other properties of the polymer are greatly affected by the monomer used in the preparation.